This section provides background information to facilitate a better understanding of the various aspects of the disclosure. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
An oil or gas well often has a subsurface section that is drilled directionally, i.e., inclined at an angle with respect to the vertical and with an inclination having a particular compass heading or azimuth. A typical procedure for drilling a directional wellbore is to remove the drill string and drill bit by which the initial, vertical section of the well was drilled using conventional rotary drilling techniques, and run in a mud motor having a bent housing at the lower end of the drill string which drives the bit in response to circulation of drilling fluid. The bent housing provides a bend angle such that the axis below the bend point, which corresponds to the rotation axis of the bit, has an inclination with respect to the vertical.
A “toolface” angle with respect to a reference, as viewed from above, is established by slowly rotating the drill string and observing the output of various orientation devices until the desired azimuth or compass heading is reached. The mud motor and drill bit are then lowered (i.e., the weight of the drill string is loaded onto the drill bit) with the drill string non-rotatable to maintain the selected toolface, and the drilling fluid pumps are energized to develop fluid flow through the drill string and mud motor. The mud motor converts the hydraulic energy of the drilling fluid into rotary motion of a mud motor output shaft that drives the drill bit. The presence of the bend angle causes the bit to drill on a curve until a desired borehole inclination has been established. Once the desired inclination is achieved at the desired azimuth, the drill string is then rotated so that its rotation is superimposed over that of the mud motor output shaft, which causes the bend section to merely orbit around the axis of the borehole so that the drill bit drills straight ahead at whatever inclination and azimuth have been established.
Various problems can arise when sections of the wellbore are being drilled with a mud motor and the drill string is not rotating. The reactive torque caused by operation of a mud motor can cause the toolface to gradually change so that the borehole is not being deepened at the desired azimuth. If not corrected, the wellbore may extend to a point that is too close to another wellbore, the wellbore may miss the desired subsurface target, or the wellbore may simply be of excessive length due to “wandering.” These undesirable factors can cause the drilling costs of the wellbore to be excessive and can decrease the drainage efficiency of fluid production from a subsurface formation of interest. Moreover, a non-rotating drill string will cause increased frictional drag so that there is less control over the “weight on bit” and the rate of drill bit penetration can decrease, which can also result in substantially increased drilling costs. Of course, a non-rotating drill string is also more likely to get stuck in the wellbore than a rotating one, particularly where the drill string extends through a permeable zone that causes significant buildup of mud cake on the borehole wall.
Rotary steerable drilling systems minimize these risks by steering the drill string while it's being rotated. Rotary steerable systems, also known as “RSS,” may be generally classified as either “push-the-bit” systems or “point-the-bit” systems.